Behaviourism (or behaviorism in American English), is a position in philosophy of mind and psychology which asserts that speak of mental states may be reduced to speak of behaviours. In other words, to the behaviourist, thinking and feeling are essentially behaviours.
For the behaviourist, the phrase “Jane is thirsty” means nothing more than “Jane is likely to drink” (or try to drink).
Behaviourism generally holds that since the mental states of other individuals are inaccessible, they have no business in either ontology or in general speak. Strong behaviourists go a step further to assert that the mind does not exist at all, and is always reducable to behaviour. Opponents of behaviourism argue that this is counter-intuitive, since each of us has access to one case in which this does not seem to be true, namely our own mind.
See: mind-body problem